Collecting the entire hair, along with at least a portion of the root is typically preferred. How the hair sample is collected may impact what information can be obtained. Cortisol is also water soluble, and hair that is subject to frequent washing may have lower cortisol levels than samples from hair that has not been washed as often. An example of this is a subject with a chronic disease, where the most recent flare-up occurred too far back in time to be observed in short or frequently shed hair samples causing the diseased and control animals to have similar cortisol levels. Additionally, if the stress event is further in the past than the growth and shed rate of the hair being sampled it may be missed. If the stressing event was truly acute and short term, like the capture and transport of an animal over the course of 1 day for immediate relocation for release, the overall difference in cortisol deposited within the hair may not be significant enough to be reliably discerned if the entire length of the hair is being used as the sample. It is also important to consider the question of whether the stressing factor being studied is chronic or acute, and when, relative to hair collection, the event occurred. Deposition of cortisol within the hair may vary both with the location of the hair on the body and with the color of the hair sampled 1,4,6,7, so consistency in sample collection is vital. There are some complicating factors that will need to be considered. Indeed, depending up the length of the hair and the rate of hair growth, it may be possible to section hair and look at stress levels in specific periods or seasons as well. Furthermore, since it has been proven cortisol is deposited in hair as the hair grows, hair cortisol levels can provide information on chronic or long term stress levels that may not be observable in the relative snap-shot provided by fecal or blood samples. Studies in a wide range of species 1-5,7 have shown that the stress hormone cortisol can be measured in hair samples.
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